1. Field of the Invention
The present invention relates to an accumulation control apparatus for photoelectric conversion elements to execute the accumulation control of the photoelectric conversion elements, which is suitably used for the visual axis (sight axis) detection apparatus, focal point detection apparatus, or the like for a camera using a sensor comprising photoelectric conversion elements, for example, and also, to a visual axis detection apparatus using such a control apparatus.
2. Related Background Art
Using photoelectric conversion elements, it is possible to obtain the electric signals corresponding to the irradiated luminous energy. However, for obtaining an optimal amount of the electric signals which can provide sufficient contrasts as required without the influence of the dark current, which is the output other than that due to irradiated luminous energy, the accumulation time must be controlled in accordance with the irradiated luminous energy within the electric signal capacitance capable of providing only a limited accumulation.
In general, therefore, when obtaining the information needed for the control of the accumulation time, the following operation should be executed as preparatory processes to secure an optimal amount of signals:
At first, an accumulation is executed for a specific period of accumulation time in advance in order to obtain an amount of signals. The amount of signals thus obtained will be read out per pixel from the photoelectric element.
Then, the signals are processed for the entire pixel outputs or for the pixels which constitute an area partially needed because the luminous energy is seldom uniformly irradiated to the photoelectric conversion elements for which the accumulation should be controlled.
Then, in an area having the optimum signal amount, the most optimal accumulation time that will be obtainable by the outputs of the electric signals of the required pixels is obtained from the average value of the pixel outputs as well as the accumulation time obtained on the basis of the results of the signal processing executed by per pixel unit, and then, the accumulation control is executed by the accumulation time thus obtained for the provision of the electric signals.
Also, in order to obtain the information required for controlling the accumulation time, it may be considered that the accumulation time is controllable by the application of the luminance information regarding an object in an optical equipment such as a camera which is provided with photometric means for measuring the luminance of the object.
In a conventional accumulation control apparatus such as mentioned above, the average value of the pixel outputs in an area required for the information thereby to control the accumulation time is read out per pixel, and provided by means of driving and signal processing for the control of the accumulation of the photoelectric conversion elements in order to obtain the optimal amount of signals. Consequently, the two-time accumulation, per-pixel read-out, and signal processing should be executed just to obtain the optimal image signals each time. Thus, it requires an enormous amount of time to complete them. Also, the dependence on pixel numbers is inevitably great, making it difficult to implement the appropriate measures for the high pixel arrangement.
Also, in utilizing this for the visual axis detection apparatus for a camera or the like, the time required for the visual axis detection occupies an extremely large ratio as compared to the other series of operational processes for the camera. As a result, not only because of the time needed for an apparatus which uses the information provided by the outputs of the photoelectric conversion elements, but also because of the lowered function of the system as a whole brought about by this imbalanced ratio of timing required to use such an accumulation control apparatus, the user feels extremely uncomfortable when operating this camera.
Now, in continuation, the description will be made of the visual axis detection apparatus.
In Japanese Patent Laid-Open application No. 1-274736, for example, the parallel luminous fluxes are projected from a light source to the anterior eye of an observer in order to obtain his visual axis by utilizing the reflection light each from the cornea reflection images and the image formation position of the pupil. Also, Japanese Patent Laid-Open Application No. 2-65835 proposes a method with which to detect the boundary between the cornea reflection images, the iris, and the pupil in an excellent precision in a stabilized condition by controlling the accumulation time per line using an apparatus capable of detecting the image information non-destructively.
FIG. 22 is a view illustrating the principle of the visual axis detection method. FIGS. 23A and 23B are views illustrating the eyeball image projected on the surface of the image sensor shown in FIG. 22, and the intensity of the output from the image sensor 14, respectively.
Now, in conjunction with FIG. 22, FIGS. 23A and 23B, the description will be made of a visual axis detection method. The infrared light emitting diodes 13a and 13b are arranged substantially symmetrical in the Z direction, respectively, with respect to the optical axis ax.sub.1 of a light receiving lens 12. These diodes illuminate the eyeball of the photographer divergently.
The infrared light emitted from the infrared light emitting diode 13b illuminates the cornea 16 of the eyeball 15. At this juncture, the cornea reflection image d formed by a part of the infrared light reflected on the surface of the cornea 16 is converged by the light receiving lens 12, hence being reimaged at the position d' on the image sensor 14.
Likewise, the infrared light emitted from the infrared light emitting diode 13a illuminates the cornea 16 of the eyeball. At this juncture, the cornea reflection image e formed by a part of the infrared light reflected on the surface of the cornea 16 is converged by the light receiving lens 12, hence being reimaged at the position e' on the image sensor 14.
Also, the luminous fluxes from the end portions a and b of the iris 17 form the images of these end portions a and b at the positions a' and b' on the image sensor 14. When the rotation angle 8 of the optical axis ax.sub.2 of the eyeball 15 is small with respect to the optical axis (optical axis ax.sub.1) of the light receiving lens 12, the coordinates Xc of the central position c of the pupil 19 is expressed as follows provided that the Z coordinates of the end portions a and b of the iris are Xa and Xb: EQU Xc.perspectiveto.(Xa+Xb)/2
Also, the Z coordinates of the center point of the cornea reflection images d and e agree with the Z coordinates Z.sub.O of the curvature center O of the cornea 16. Therefore, given the X coordinates at the positions d and e at which the cornea reflection images are generated as Xd and Xe; the standard distance from the curvature center 0 of the cornea 16 to the center C of the pupil 19 as L.sub.oc ; and the coefficient with which to consider the individual difference with respect to the distance L.sub.oc as A1, the rotation angle .theta. of the optical axis ax.sub.2 of the eyeball almost satisfies the following relational equation: EQU (A1.times.L.sub.OC).times.sin.theta..perspectiveto.Xc-(Xd+Xe)/2(1)
Consequently, as shown in FIG. 23B, the rotation angle .theta. of the optical axis ax.sub.2 of the eyeball can be obtained in the visual axis arithmetic processing unit by detecting the positions of the respective characteristic points (the cornea reflection images d and e, and end portions a and b of the iris) projected on a part of the image sensor. In this case, the equation (1) can be rewritten as follows: EQU .beta.(A1.times.L.sub.OC).times.sin.theta..perspectiveto.(Xa'+Xb')/2-(Xd'+X e')/2 (2)
where the .theta. is a magnification determined by the position of the eyeball with respect to the light receiving lens 12. Essentially, this magnification is obtainable as the coefficient of the interval .vertline.Xd'-Xe'.vertline. of the cornea reflection images. The rotation angle .theta. of the eyeball can be rewritten as follows: EQU .theta..perspectiveto.ARCSIN{(Xc'-Xf')/.beta./(A1.times.L.sub.OC)}(3)
where EQU Xc'.perspectiveto.(Xa'+Xb')/2 EQU Xf'.perspectiveto.(Xd'+Xe')/2
Now, since the optical axis ax.sub.2 of the photographer's eyeball does not agree with the visual axis, the angular correction .delta. is made between the optical axis and visual axis when the rotation angle .theta. of the optical axis ax.sub.2 of the photographer's eyeball in the horizontal direction is calculated, thus making it possible to obtain the photographer's visual axis .theta.H in the horizontal direction. Given the coefficient with which to consider the individual difference in the corrected angle .delta. between the optical axis ax.sub.2 and the visual axis as B1, the photographer's visual axis .theta.H in the horizontal direction can be obtained as follows: EQU .theta.H=.theta..+-.(B1.times..delta.)
Here, assuming that the rotation angle to the right is positive in relation to the position of the photographer, the sign + of the .+-. is selected if the photographer uses his left eye in looking in the observation equipment, and the sign - is selected if he uses his right eye for the purpose.
Also, in FIG. 23B, an example is shown, in which the photographer's eyeball is rotated in the Z-X plane (horizontal plane, for example), but the detection is equally possible for the photographer's eyeball which rotates in Z-plane (vertical plane, for example). However, since the component of the photographer's visual axis in the vertical direction agrees with the component .theta.' of the eyeball in the vertical direction, the visual axis .theta.V is: EQU .theta.V=.theta.'
Further, the positions (Xn, Yn) on the imaging plate in the finder field which the photographer looks in can be obtained by the visual axis data .theta.H and .theta.v as follows: ##EQU1## where the m is a constant determined by the finder optical system of a camera.
Here, the values of the coefficients A1 and B1 with which to correct the individual difference in the photographer's eyeball can be obtained when the photographer is requested to gaze at the indications which are arranged at given positions in the finder of a camera, and then, the positions of the indications are allowed to match the positions of the gazing points calculated by the equation (5).
In the present embodiment, the operation to obtain the photographer's visual axis and target points is executed in accordance with each of the foregoing equations by the application of the software with the microcomputer of the visual axis arithmetic processing unit.
Also, the coefficients with which to correct the individual difference in the visual axis usually correspond to the rotation of the observer's eyeball in the horizontal direction. Therefore, the two indications which are arranged in the finder of a camera are set in the horizontal direction with respect to the position of the observer.
When the coefficients for the correction of the individual difference in the visual axis are obtained, the equation (5) is used to calculate the position of the visual axis on the imaging plate of the photographer who looks in the finder of a camera. The visual axis information thus obtained is utilized for the focus adjustment of a photographing lens, exposure control, or the like.
Now, such a visual axis detection apparatus is installed in an optical apparatus like a camera or a telescope which is used both indoors and outdoors, there is a case where the observer's eyeball is illuminated by the external light (the sunlight) or it is not illuminated by such a light at all. Thus, the brightness of the eyeball image changes significantly. The dynamic range of an image sensor currently in use is considerably narrower than the ratio between the indoor and outdoor brightness for detecting the eyeball image. Hence, there is a need for the installation of the so-called auto-gain control (AGC) which enables the driving conditions of the image sensor to vary depending on brightness. In this respect, a proposal has been made to detecting the brightness of the eyeball portion by a sensor arranged in the vicinity of a finder in a patent application, Ser. No. 815,045 (filed on Dec. 31, 1991 in USA), but in an outdoor use, for example, the sunlight illuminates the eyeball locally or when the observer uses his spectacles, the source light from the sunlight or a lamp is reflected by the spectacles to illuminate the eye portion like a dot. Due to such an illumination, the brightness of the eyeball portion cannot be detected exactly. In such a case, the eyeball portion tends to be determined to be brighter than it actually is. As a result, the auto-gain controller makes the output of the image signals smaller. Then, the problem is encountered that it becomes impossible to detect the characteristic points of the eyeball image which are described in conjunction with the foregoing principle of the visual axis detection, particularly the end portions of the iris. As a result, the visual axis detection cannot be executed eventually. Also, in use in the shade or the like on the contrary, there often exist a portion which is slightly brighter due to light from the surroundings locally, and a portion which is extremely dark such as the shade of the pupil, the corners of the eye. Thus, contrary to the example mentioned above, the output of the image signals tends to be increased more than necessary. In this case, the noise of the external light is often amplified, resulting in the problem that the cornea reflection images are erroneously detected.